1. Field of the Invention
This invention relates to an optical waveguide to produce an ultra broadband optical gain which can be used for broadband optical amplifiers and ultra-broadly tunable lasers in optical wavelength division multiplexing (WDM) network systems.
2. Description of Related Art
Semiconductor optical amplifiers and tunable lasers need broad band gain in order to fully utilize the advantage of optical WDM network systems. Quantum wells have been used in these devices to provide higher and broader optical gain. Multi-quantum (MQW) wells consisting of an identical composition and thickness can provide a typical gain spectrum of 60-70. Conventional methods have been used to broaden the gain spectrum beyond this range, see S. Ikeda et al., “Semiconductor laser devices with a plurality of light emitting layers having different bands gaps and methods for driving the same” IEEE Photonics Technology Letters, Vol. 16, No. 2, February 2004, and U.S. Pat. No. 5,224,114. Ikeda et al. describe light emitting layers of two different quantum wells having different band gaps which provide two different gain peak wavelengths. Since the total gain spectrum is a sum of the two gain spectra, the gain spectrum is broader than that obtained by using one type quantum well. Ikeda et al. describe AlGaAs/GaAs materials which have a wavelength range around 850 nm. However, Ikeda et al. has the following disadvantage. There is a barrier layer between the two different quantum wells, which has a larger band gap than the band gap of the two quantum wells. In order for the barrier layer to confine the carriers in the two adjacent quantum wells, the band gaps of the two quantum well materials cannot be too different because the carrier injection efficiency degrades.
In current WDM metro and long-haul optical communications systems, 1.3 μm and 1.5 μm wavelength ranges are used. In these wavelength ranges, a broad optical gain spectrum ranging from 1.3 μm to 1.5 μm has been required. C. C. Huang et al., “174-nm mode spacing in dual-wavelength semiconductor laser using non-identical InGaAsP quantum wells”, IEEE Photonics Technology Letters, vol. 16, No. 2, pp. 371-373, February 2004. Using InGaAsP and InGaAs quantum wells to provide gain peak wavelengths of around 1.3 μm and around 1.5 μm, respectively. Accordingly, a broad gain spectrum range from 1356 nm to 1530 nm is achieved. However, Huang et al. has the following disadvantage. In order to broaden the gain spectrum band, the difference of the band-gaps of the two quantum wells materials needs to be larger. Since the barrier layer of InGaAsP is used for the two quantum wells, the larger band gap difference in the two quantum wells causes a degradation of carrier injection efficiency, resulting in the reduction of the magnitude of the gain.
It is desirable to provide an optical waveguide with improved higher and broader optical gain.